Most flame sensors must have a way to monitor the health of the optical system. Without this and if the window gets coated, a flame could occur and the sensor would be totally unaware. To overcome this problem some IR sensors have an integral light source. The internal light source is either inside a sensor housing or outside the sensor housing. Light from the internal light source is projected through optics to prove that the sensor is still imaging and that a lens or optics is not contaminated. The main purpose is to monitor degradation of transmission over an extended time (weeks, months) rather than any instant change. The use of such a light source can draw additional power, which may shorten sensor battery life. In addition, the intensity of the light source itself may vary over time, either from gradual failure or decrease in power, causing an error in monitoring the health of the optical system.